Recently, the High Pressure Acid Leach: HPAL by using sulfuric acid has been gathering attention, as a hydrometallurgical process for nickel oxide ore. This process does not include dry steps, such as a reduction step and a drying step, unlike the pyrometallurgy process, that is a conventional common metallurgical process for nickel oxide ore, but comprises a continuous wet process. Thus, it is advantageous in regards to energy and cost. Additionally, the process has the advantage of capable of providing a sulfide containing nickel and cobalt whose nickel grade is upgraded up to about 50% by weight or so (hereinafter, referred sometimes to as a mixed nickel-cobalt sulfide).
The hydrometallurgical process for nickel oxide ore by utilizing this HPAL process comprises, for example, the following steps. Namely, a leaching step of leaching slurry, under high temperature and high pressure, by adding sulfuric acid to slurry of the nickel oxide ore to produce leached slurry; a solid-liquid separating step of separating residue while multistage cleaning the leached slurry to produce leachate containing impurity elements together with nickel and cobalt; a neutralization step of separating neutralized sediment containing the impurity elements by controlling pH of the leachate produced by separating the residue to produce a post-neutralization solution containing zinc together with nickel and cobalt; a dezincification step of producing a zinc sulfide by adding a hydrogen sulfide gas to the post-neutralization solution and separating the zinc sulfide to produce a mother liquid for nickel recovery containing nickel and cobalt; and a nickel recovery step of producing a mixed sulfide containing nickel and cobalt by adding the hydrogen sulfide gas to the mother liquid for nickel recovery to separate a mixed nickel-cobalt sulfide.
Herein, in the above-mentioned neutralization step in the hydrometallurgical process, for example, the leachate produced from the solid-liquid separating step is introduced into a neutralization tank where the leached is neutralized by adding calcium carbonate slurry, and a produced hydroxide sediment is solid-liquid separated to produce a neutralized sediment and a post-neutralization solution.
In the dezincification step, the post-neutralization solution is introduced into a sulfurization reaction tank where zinc and copper or the like, contained in the post-neutralization solution is sulfurized by adding a sulfurizing agent, such as a hydrogen sulfide gas and a sodium hydro sulfide, followed by solid-liquid separation of a sulfurized post-neutralization solution with a filter press or the like, to produce the zinc sulfide and the mother liquid for nickel recovery containing nickel and cobalt (for example, see Patent Documents 1 and 2).
Incidentally, since the mixed nickel-cobalt sulfide produced by this hydrometallurgical process is further used as raw materials for purifying even electronic nickel and electronic cobalt, it is required for Zn concentration in the post-solution to reduce to 1 mg/L or less, in the dezincification step.
In the dezincification step, it is desirable to prevent the occurrence of clogging in a filter cloth to suppress a reduction in a filtration rate in filtrating and separating the produced zinc sulfide by using the filter cloth.
As a method for preventing the clogging in the filter cloth, it has been proposed hitherto a technique of causing suspended solid composed of the neutralized sediment and the leached residue to be remained, and improving the filterability of the filter cloth, with pH of the post-neutralization solution produced in the above-mentioned neutralization step adjusted to 3.0 to 3.5, and the turbidity of the post-neutralization solution to 100 to 400 NTU (for example, see Patent Document 3).
However, it is true the technique disclosed in the Patent Document 3 undoubtedly succeeded in substantially improving a filtration rate, and decreasing the frequency of clogging in the filter cloth, as compared with the prior art in those days, but the clogging is still occurring in the filter cloth. On that account, it has been under the necessity to take any measure of restoring the filtration rate of the filter cloth to its original good condition by appropriately performing a cleaning operation of the clogged filter cloth. If once clogging occurs stubborn to the extent that the effect accomplished by the cleaning operation wears off, the filter cloth reaches its service life and one is forced to replace the clogged filter cloth with new one.
Cleaning and replacing operations of the filter cloth in such dezincification step entail a closedown of the neutralization plant and they compel one to do heavy labor. Hence, it has been longing thus far for the advent of a method for more effectively preventing the occurrence of the clogging in the filter cloth, and prolonging a service life of the filter cloth.